This invention concerns a pigment dispersant, more particularly, it concerns such a dispersant in which the backbone is made by group transfer polymerization processes.
AB dispersants are known having typically a polar group known as the A segment on one end to facilitate attachment to a pigment surface, and a non-polar polymeric or oligomeric group known as the B segment on the other end to enhance steric stabilization of the pigment particles in a dispersion. See U.S. Pat. Nos. 4,032,698 disclosing acrylics with substituted urea end groups; 4,029,861 on polyesters with poly carboxylic acid end groups and 3,788,996; 4,035,557; 4,048,207; and 4,070,388.
British Pat. No. 1,108,261 (1963) and counterpart U.S. Pat. No. 3,433,753 (1969) teach using free radical techniques to prepare dispersant compositions with randomly repeating AB groups including polymers with a backbone of methyl methacrylate (MMA) and glycidyl methacrylate (GMA), with pendant groups of p-nitrobenzoic acid reacted with, for instance 12% (Example 1 of the British Patent) or 90% (Example 10) of the epoxy groups. The backbone is a random copolymer with a molecular weight of about 80,000. Solvents used were polar methyl isobutyl ketone (MIBK) and much larger proportions of non-polar toluene. Such dispersants are useful, but to obtain such high molecular weights, little if any chain transfer agents are used. This results in a large polydispersity of sizes and degree of functionality of the backbone portion. The low proportion of epoxy groups reacted with polar groups can give shelf stability problems and contributes to flocculation. Also, such large polymers tend to provide random large loops sticking out from the pigment surface between the polar group points of attachment. This is a less efficient way to obtain steric stabilization of a pigment dispersion than if the B segments were shorter and of more uniform size, as in a simple AB dispersant without repeating AB sequences. U.S. Pat. No. 4,388,448--Melby discusses low temperature anionic polymerization to make epoxy block copolymers.
Group transfer polymerization (GTP) is a known and established method of acrylic polymerization in which an initiator splits with one end going to the tail of an acrylic monomer unit and the other to the head. The group at the head transfers to the head of a second monomer unit as that unit is polymerized with the first. Such techniques are described at length in various U.S. patents.
O. W. Webster, U.S. Pat. Nos. 4,417,034 (Nov. 22, 1983) and 4,508,880 (Apr. 2, 1985), and W. B. Farnham and D. Y. Sogah, U.S. Pat. Nos. 4,414,372 (Nov. 8, 1983) and 4,524,196 (June 18, 1985) showed that acrylic-epoxy polymers can be prepared via group transfer polymerization. GTP techniques have also been described in the independent scientific literature including F. Bandermann and H. Speikamp (of the Institute fur Technische Chemie, Universitat-Gesamthochschuls Essen) "Group Transfer Polymerization of Methyl Methacrylate" Makromol. Chem., Rapid Commun. 6, 335-339 (1985). Oxyanion catalysts for GTP processes are described in U.S. application Ser. No. 707,193 Dicker et al., filed Mar. 1, 1985, now U.S. Pat. No. 4,588,795. However, the prior GTP patents, application and literature apparently do not suggest ways to improve known AB dispersants. Catalyst life enhancement can be obtained using the teachings of U.S. application Ser. Nos. 707,192 and 707,194--both Dicker et al., filed Mar. 1, 1985, both abandoned.
The above-identified patents and the application on AB dispersants and GTP processes are hereby incorporated herein by reference.